Preheat oven to 250°F. Halve tomatoes through stem ends and arrange, cut sides up, in 1 layer in a large shallow (1-inch-deep) baking pan. Add garlic to pan and roast in middle of oven until tomatoes are slightly shriveled around edges, about 1 hour. Cool in pan on a rack 30 minutes. Peel garlic and puree with oil, water, lemon juice, salt, pepper, and 1/2 cup roasted tomatoes in a blender until dressing is very smooth. Bring broth to a boil in a 3-quart heavy saucepan and stir in couscous, then simmer, uncovered, 6 minutes. Cover pan and remove from heat. Let stand 10 minutes. Spread couscous in 1 layer on a baking sheet and cool 15 minutes. Transfer couscous to a bowl and stir in remaining ingredients, dressing, roasted tomatoes, and salt and pepper to taste.